1. Field of the Invention
The present invention relates to a process for the preparation of semiconductor device. More specifically, the present invention relates to a process for improving the electrical characteristics of amorphous silicon to be used for a photodiode or the like.
2. Description of the Related Art
A gap cell type photodiode shown in FIG. 3 is known. In FIG. 3, reference numeral 11 represents a glass substrate, reference numeral 12 represents an amorphous silicon film (a-Si:H film), reference numeral 13 represents an Al electrode, and reference numeral 14 represents a white light of, for example, 1000 lx.
An apparatus for depositing an amorphous silicon film is shown in FIG. 2. In FIG. 2, reference numeral 21 represents a reaction furnace (quartz jar), reference numeral 22 represents a heater, reference numeral 23 represents an electrode, reference numeral 24 represents a radio frequency (RF) generator, reference numeral 25 represents a gas-introducing pipe, reference numeral 26 represents an exhaust opening, and reference numeral 28 represents a susceptor. The heater 22 is disposed to heat the glass substrate 11 on which an amorphous silicon film is deposited. The substrate and one end of the RF generator 24 are grounded, and when the RF generator 24 is turned on, plasma 27 indicated by dots in FIG. 2 is generated between the electrode 23 and the substrate 11, and Si of a silane gas (SiH.sub.4) introduced through the gas-introducing pipe 25 is precipitated and deposited on the glass substrate 11 to form an amorphous silicon film (a-Si:H). A diluent gas such as H.sub.2 or Ar may be added to SiH.sub.4. It is understood that the structure of a so-prepared amorphous silicon is as follows: ##STR1##
The plasma chemical vapor deposition process (CVD process) using an exciting high frequency wave of 13.56 MHz has been adopted for formation of an amorphous silicon layer (e.g., Japanese Journal of Applied Physics, Vol. 21, No. 3, March, 1982, pp. 413-417). If this high frequency wave is used, the number of dangling bonds is small and amorphous silicon having excellent electrical characteristics [dark electroconductivity .sigma.d is small and bright electroconductivity (photoconductivity) .sigma.p is large] is easily obtained. However, this process is defective in that the formed amorphous silicon has a poor heat durability (below 300.degree. C.) and adhesion to the glass substrate as the base.
On the other hand, when an exciting wave of a frequency of less than 1 MHz is used, an amorphous silicon film having an excellent heat durability and adhesion can be obtained, but the above-mentioned electrical characteristics are poor.
If the heat durability is poor at a temperature lower than 300.degree. C., the preparation of a device becomes difficult. A high heat durability is an indispensable requirement for the preparation process, and the adhesion to the substrate is necessary for preventing the amorphous silicon film from peeling from the substrate. Moreover, a smaller .sigma.d and larger .sigma.p are preferred for the electrical characteristics.
The foregoing are summarized in Table 1.
TABLE 1 ______________________________________ High Frequency Low Frequency ______________________________________ Heat Durability .DELTA. o Adhesion to Substrate .DELTA. o Electrical Characteristics o .DELTA. ______________________________________
In Table 1, "o" indicates good and ".DELTA." indicates bad.